On some receipts, securities, and certificates, a special pattern which emerges as a character string or image when they are copied is printed on the background, so as to prevent them from being readily copied. Such special pattern is generally called a “copy-forgery-inhibited pattern”, which applies a mechanism that does not allow an original to be readily copied, thus psychologically deterring a copy of an original.
This copy-forgery-inhibited pattern is formed of two regions having an identical density, i.e., a region where dots remain after copying and a region where dots disappear after copying. These two regions have substantially the same densities, and a hidden character string or image such as “COPY” cannot be seemingly recognized macroscopically, but these regions microscopically have different properties. Note that the hidden character string or image will be referred to as a “latent-image” hereinafter.
For example, the region where dots remain after copying (this region will be referred to as a latent-image part or region hereinafter) is formed of groups of dots where dots are concentrated, and the region where dots disappear or become lighter than the latent-image part after copying (this region will be referred to as a background-image part or region hereinafter) is formed of scattered dots. In this manner, the two regions which have substantially the same densities but different properties can be created.
The concentrated dots and scattered dots can be generated by halftoning using halftone dot screens with different screen ruling values or dithering using dither matrices having different features as an image process.
In halftoning, a halftone dot screen with a low screen ruling value is preferably used to obtain the concentrated dot layout, and a halftone dot screen with a high screen ruling value is preferably used to obtain the scattered dot layout.
In the dithering using a dither matrix, a dot-concentration dither matrix is preferably used to obtain the concentrated dot layout, and a dot-scattering dither matrix is preferably used to obtain the scattered dot layout.
Therefore, when a copy-forgery-inhibited pattern image is generated using the aforementioned halftoning, halftoning with a low screen ruling value is suited to a latent-image part, and halftoning with a high screen ruling value is suited to a background-image part. When a copy-forgery-inhibited pattern image is generated using the aforementioned dithering, dithering using a dot-concentration dither matrix is suited to a latent-image part, and dithering using a dot-scattering dither matrix is suited to a background-image part.
In general, a copying machine suffers a limitation on image reproduction performance depending on the input resolution upon reading small dots of a document to be copied or the output resolution upon reproducing small dots. Therefore, when a document includes isolated small dots beyond the limitation on the image reproduction performance of the copying machine, these small dots cannot be perfectly reproduced on its copy, and the isolated small dots disappear.
That is, when the background-image part which forms the copy-forgery-inhibited pattern image is designed to exceed the limitation of dots that can be reproduced by the copying machine, large dots (concentrated dots) of the copy-forgery-inhibited pattern can be reproduced by copying, but small dots (scattered dots) cannot be reproduced. Hence, a hidden image (latent-image) emerges. Also, even when scattered dots do not completely disappear after copying, if they have an apparent density difference from concentrated dots after copying, a hidden image (latent-image) emerges.
In the copy-forgery-inhibited pattern, a technique called “camouflage” which makes it harder to recognize a hidden character string or image as a latent-image is well known. This camouflage technique is a method of laying out a pattern which has a density different from that of the latent-image part and background-image part on the entire copy-forgery-inhibited pattern image. This technique has an effect of macroscopically emphasizing the camouflage pattern with a density different from that of the latent-image part and background-image part, and further obscuring a latent-image at a glance.
The copy-forgery-inhibited pattern with the camouflage pattern has an effect of giving a decorative impression to a print compared to a copy-forgery-inhibited pattern without any camouflage pattern. Dots inside the camouflage pattern preferably disappear as much as possible so as to allow easy recognition of a latent-image after copying. In case of the simplest implementation, camouflage can be realized by printing no dots at positions corresponding to the camouflage pattern.
An overview of the copy-forgery-inhibited pattern has been explained.
Conventionally, a print paper vendor prints a copy-forgery-inhibited pattern including a character string or image (latent-image) such as “COPY” or the like on dedicated sheets, and sells such sheets as copy-inhibition paper sheets. The government and other public offices, and companies buy such copy-inhibition paper sheets, and print documents whose authenticity is to be guaranteed on copy-inhibition paper sheets, thus deterring copies of prints.
Since the aforementioned copy-inhibition paper sheets are marketed as pre-print sheets by pre-printing a copy-forgery-inhibited pattern image on dedicated sheets by a print paper vendor, users have demerits in terms of cost such as cost produced upon using dedicated sheets, cost produced upon preparing pre-print sheets more than necessary, and the like.
By contrast, in recent years, a technique for creating a copy-forgery-inhibited pattern image by software, and outputting a composite document of that copy-forgery-inhibited pattern image and contents image using a laser printer (to be referred to as “on-demand copy-forgery-inhibited pattern output method by printer” hereinafter) has been realized (e.g., see patent reference 1: Japanese Patent Laid-Open No. 2001-197297).
With this on-demand copy-forgery-inhibited pattern output method by a printer, since a document with a copy-forgery-inhibited pattern image can be printed using plain paper, only a required number of documents with a copy-forgery-inhibited pattern image on their backgrounds can be printed when needed. Therefore, copy-inhibition paper sheets need not be prepared more than necessary unlike in the conventional method. That is, the on-demand copy-forgery-inhibited pattern output method by a printer can greatly reduce cost of sheets compared to the conventional document copy-deterrence method using copy-inhibition paper sheets.
The user of the conventional copy-inhibition paper sheets can use only a character string or image (latent-image) prepared in advance by the print paper vendor or a made-to-order hidden character string or image (latent-image).
However, with the on-demand copy-forgery-inhibited pattern output method by a printer, the user can generate a copy-forgery-inhibited pattern image including an arbitrary hidden character string or image (latent-image) by a software process for each print, and can print it on-demand using a printer. Hence, the user can freely customize a hidden character string or image (latent-image).
By exploiting a merit of on-demand selection of a latent-image, not only a corporation logo mark or a character string “VOID”, which is used conventionally, but also various kinds of information such as a serial number or IP address used to identify an output printer, a computer name or IP address used to identify a computer that issues a print command, a user name or login name used to identify a user who issues a print command, a print job number, print date, print location, the file name of a digital document, and the like used to identify when and by whom a print process is done, and so forth can be selected as an image or character string to be embedded as a latent-image.
As a result, the on-demand copy-forgery-inhibited pattern output method by a printer can implement an advanced tracking function that cannot be implemented by the conventional pre-printed copy-inhibition paper sheets.
In the on-demand copy-forgery-inhibited pattern output method by a printer, the generation timing of a copy-forgery-inhibited pattern image is processed mainly using a personal computer (PC) or workstation, and a printer controller for some processes. However, computers and printers include various models, i.e., from a model having high computation performance and a sufficiently large memory size to a model having poor computation performance and a small memory size.
Even if a computer and printer have sufficiently high performance, assuming that a high-resolution copy-forgery-inhibited pattern image is to be generated in large quantities, the computation volume and memory size required to generate a copy-forgery-inhibited pattern image are preferably reduced as much as possible.
Patent reference 1 that describes the on-demand copy-forgery-inhibited pattern output method by a printer also describes a method of reducing the computation volume and memory size upon generation of a copy-forgery-inhibited pattern image. However, patent reference 1 can achieve reductions of the computation volume and memory size in some processes, but cannot achieve both reductions of the computation volume and memory size and high image quality.
For example, patent reference 1 describes a method of executing a process for separating a color main image as contents such as a document or the like into color components such as Y, M, C, and K or the like, and compositing a copy-forgery-inhibited pattern image as a monochrome image in a designated color component of the main image upon compositing the main image and copy-forgery-inhibited pattern image.
However, for example, if a color component in which the copy-forgery-inhibited pattern image is to be embedded is C (cyan) and the copy-forgery-inhibited pattern image is to be composited to the C component of the color main image, only when other color components of the color main image are zero, the color of the color main image composited with the copy-forgery-inhibited pattern image is output as cyan. Otherwise, cyan is mixed with other color components and a mixed-color copy-forgery-inhibited pattern may be output (for example, mixing of cyan and yellow generates green).
Patent reference 1 does not describe any method of compositing a copy-forgery-inhibited pattern image in an accurate color to only a designated region (coordinate position, color component) of an input image.
In patent reference 1, a camouflage pattern and background image are integrated in advance, and error diffusion is applied to the integrated background image with the camouflage pattern to generate a binarized background image. However, the camouflage pattern in the background image binarized by error diffusion may suffer positional deviation from that in a multi-valued background image.
Error diffusion is a method of ON/OFF of a dot by comparing the sum of the pixel value of a pixel to be binarized and errors distributed from surrounding pixels to that pixel value with a predetermined threshold value. However, error diffusion suffers known problems, e.g., “delay of dot generation” (errors are sufficiently accumulated in black dot generation at the leading end of a low-density region or white dot generation at the leading end of a high-density region and dot generation delays considerably until dots are fixed down to a steady state), “excessive diffusion” (errors accumulated in large quantities are diffused to outside a region), and the like.
Therefore, even when error diffusion is applied to a background image with a camouflage pattern to generate a binary background image with a camouflage pattern, “delay of dot generation” and “excessive diffusion” may occur in pixels around the camouflage pattern.
When halftoning is applied to a latent-image part designated by a mask image, e.g., ON/OFF of a dot is determined by comparing the pixel value of a background image and a threshold matrix value, “delay of dot generation” and “excessive diffusion” as problems of error diffusion do not occur.
Therefore, a camouflage pattern obtained as a result of error diffusion may not match that as a result of halftoning at their boundaries.
Patent reference 1 also describes a method of applying halftoning to a latent-image part to binarize it, and binarizing the entire background image including the latent-image part by error diffusion. However, in this case, dots generated by error diffusion get into the latent-image region to disturb the boundary between the latent-image region and background region, thus impairing the quality of the copy-forgery-inhibited pattern image.
Therefore, the conventional method does not sufficiently consider any efficient computation volume/memory size reduction method upon generation of a copy-forgery-inhibited pattern image while maintaining high image quality of the copy-forgery-inhibited pattern image.
As described above, many efforts have been made in the copy-forgery-inhibited pattern to make it harder to recognize a latent-image. The camouflage technique is one of such efforts.
However, the on-demand copy-forgery-inhibited pattern output method by a printer is susceptible to density variations of a printer. In addition to the camouflage technique, efforts that make it harder to recognize a latent-image are required even when the density variations of a printer have occurred.